1. Field of the Invention
The present invention relates to a display apparatus. More particularly, the present invention relates to a picture display apparatus which has the function of displaying a video signal with a number of vertical pixels different from the number of vertical pixels of a display unit of the picture display apparatus as an enlarged or thinned picture.
2. Description of the Related Art
Typically, in a picture display apparatus for a personal computer, the number of pixels on a display panel is prescribed by specifications such as VGA specifications, SVGA specifications, XGA specifications, SXGA specifications and UXGA specifications where VGA, SVGA, XGA, SXGA and UXGA are IBM""s registered trademarks indicating commonly known representative specifications. In some cases, however, the number of pixels of a video signal supplied to a picture display apparatus may be different from the number of pixels on a display panel of the apparatus. For example, a picture conveyed by a VGA video signal is displayed on a display panel conforming to the XGA specifications. In such a case, it is necessary to display the video signal on the display panel as an enlarged or thinned picture.
When a video signal is displayed in the conventional system as a picture enlarged in the vertical direction, for example, data to be displayed on the area as an enlarged picture is stored in a memory and the data is then written onto a plurality of lines on the picture display apparatus. In this conventional system, however, peripheral components such as the memory and an A/D converter are required, making the picture display apparatus large in size and complicated. In order to solve this problem, there has been proposed the following method of displaying a video signal on a display panel as an enlarged picture without employing such peripheral components.
In a picture display apparatus having a function of displaying a video signal on a display panel thereof as an enlarged picture, a mode signal is set in a gate driver to indicate whether a video signal is to be displayed on the display panel normally or as an enlarged picture. During a horizontal period for outputting picture data of a line, driving of a gate line and driving of a plurality of gate lines are switched from one to another in dependence on the type of the mode signal. A gate line driven during a horizontal period will result in a normal display. On the other hand, a plurality of gate lines driven at the same time during a horizontal period will display picture data of a line on the same plurality of lines on the display screen, resulting in a display enlarged in the vertical direction. FIG. 8 is a timing diagram showing the operation of a gate driver employed in the picture display apparatus. FIG. 8A is a timing diagram for the normal mode while FIG. 8B is a timing diagram for an enlarge mode with a multiplication factor of two. As shown in FIG. 8B, gate output waveforms on two adjacent lines, namely X1 and X2 or X3 and X4, are generated at the same time.
In equipment such as a liquid-crystal display apparatus, there is generally adopted a technique whereby a supplementary capacitor is added to each pixel to hold electric charge during each scanning period. As a technique to lower the numerical aperture without using a capacitive electrode for forming a supplementary capacitor, a pixel electrode and a gate line are put in an overlapping layout to form a structure of a supplementary capacitor known as the so-called Cs on-gate structure. That is, the supplementary capacitor has a configuration comprising the pixel electrode and the gate line.
However, the enlarged-picture display technology can not be applied to a liquid-crystal display apparatus provided with supplementary capacitors each having the Cs on-gate structure. This is because, in the Cs on-gate structure, a gate line adjacent to a gate line driving a pixel serves as one of the electrodes of a supplementary capacitor of the pixel so that, when data is written into a pixel connected to one of the gate lines, that is, when the waveform output by the gate is set at a high level, the supplementary capacitor does not function unless the waveform output by a gate connected to the adjacent gate line is set at a low level. Since the technique to display an enlarged picture causes gates connected to two adjacent gate lines to output the same waveform, the supplementary capacitor does not function.
It is thus an object of the present invention addressing the problem to provide a display device which functions to display an enlarged picture and functions to display a shrunk (thinned) picture and is thus applicable to a picture display apparatus such as a liquid-crystal display apparatus provided with supplementary capacitors each having the Cs on-gate structure.
In order to achieve the object described above, the present invention provides a display apparatus that functions to display an enlarged picture. The display apparatus is characterized in that the apparatus is provided with a driving circuit having: a pulse generating means for generating second clock pulse signals to copy in a horizontal period in an operation to display a video signal having a vertical-pixel count smaller than a predetermined vertical-pixel count of a display unit as an enlarged picture on a screen of the display unit by using two kinds of fields in addition to as many original clock pulse signals as required in an operation to display a video signal having a vertical-pixel count equal to the predetermined vertical-pixel count of the display unit and for repeating generation of the original clock pulse signals and the second clock pulse signals for each horizontal period; a gate-clock generating means for receiving the original clock pulse signals and the second clock pulse signals from the pulse generating means and for generating gate-clock signals obtained as a result of superposing all of the original clock pulse signals on some of the second clock pulse signals wherein the number of the superposed second clock pulse signals is equal to a difference obtained as a result of subtraction of the vertical-pixel count of the video signal from the predetermined vertical-pixel count of the display unit and any adjacent two of the superposed second clock pulse signals pertain to the two kinds of fields separated from each other by at least two horizontal periods; and a gate driving means for receiving the gate clock signals from the gate-clock generating means and for generating gate driving signals used for driving as many gate lines as pulses of the gate clock signals for each horizontal period.
In a driving circuit employed in a display apparatus of the present invention having a function to display an enlarged picture, in the first place, a pulse generating means generates an original clock pulse signal and a second clock pulse signal for a copy purpose during a horizontal period. These clock pulse signals are generated repeatedly for each horizontal period. In order to generate gate clock signals having a number of pulses matching the vertical-pixel count of the display unit, the gate-clock generating means is used for generating gate-clock signals obtained as a result of superposing all of the original clock pulse signals on some of the second clock pulse signals wherein the number of the superposed second clock pulse signals is equal to a difference obtained as a result of subtraction of the vertical-pixel count of the video signal from a predetermined vertical-pixel count of the display unit. Then, the gate driving means receives the gate clock signals from the gate-clock generating means and generates a plurality of gate driving signals corresponding to pulses of the gate clock signals. The gate driving signals are generated with timings different from each other, being set at a high level for equal periods of time. It should be noted that a high-level period is a period between the rising edge of a pulse and the rising edge of the next pulse.
In such an operation, a plurality of gate lines are thus driven during a horizontal period. As a result, since the same video data of a line is displayed on a plurality of lines on the display unit, an enlarged picture matching the number of vertical pixels on the display unit is displayed thereon.
At that time, a plurality of gate driving signals driving the same plurality of gate lines are set at a high level with timing different from each other. Thus, unlike the conventional technique used to display an enlarged picture, two adjacent gate lines are never set at a high level at the same time. As a result, the technique provided by the present invention can be applied to a liquid-crystal display apparatus provided with supplementary capacitors each having a Cs on-gate structure without problems.
In addition, according to the present invention, in superposition of all of the original clock pulse signals on some of the second clock pulse signals, any adjacent two of the superposed second clock pulse signals pertain to the two kinds of fields separated from each other by at least two horizontal periods. The reason is described as follows. Assume that, unlike the configuration of the present invention, any adjacent two of the superposed second clock pulse signals pertain to the two kinds of fields not separated from each other by at least two horizontal periods in a display apparatus having a screen comprising the two kinds of fields, namely, odd and even fields. For example, two superposed second clock pulse signals are generated in adjacent horizontal periods along the time axis. In this case, a video signal of an odd field (or an even field) is copied into a specific gate line while a video signal of an even field (or an odd field) is copied into another gate line adjacent to the specific gate line.
Assume that the video signal in the state represents for example a slanting line to be displayed on the screen. In this case, a portion obtained as a result of the copy operation using the adjacent gate lines become striking in comparison with other portions. To the eyes of the user, the striking portion appears as a thick segment of the line or an unevenness. In the case of the present invention, on the other hand, any adjacent two of the superposed second clock pulse signals pertain to the two kinds of fields separated from each other by at least two horizontal periods. Thus, a video signal of an odd field (or an even field) is copied into a specific gate line while a video signal of an even field (or an odd field) is copied into another gate line separated away from the specific gate line. As a result, no striking portion appears as a thick segment of the line or an unevenness to the eyes of the user.
On the other hand, a display apparatus having a function to display a thinned picture is characterized in that the display apparatus is provided with a driving circuit having: a pulse generating means for displaying a screen on a display unit by using two kinds of fields by writing a video signal in a horizontal period into two adjacent gate lines of the display unit wherein, in an operation to display a video signal having a vertical-pixel count greater than a predetermined vertical-pixel count of the display unit, the video signal is thinned by generating a second clock pulse signal for a thinning purpose in a horizontal period in addition to two original clock pulse signals as required in an operation to display a video signal having a vertical-pixel count equal to the predetermined vertical-pixel count of the display unit and by repeating generation of the original clock pulse signals and the second clock pulse signals each having a pulse width equal to that of each of the original clock pulse signals for each horizontal period; a gate-clock generating means for receiving the original clock pulse signals and the second clock pulse signals from the pulse generating means and for generating gate-clock signals obtained as a result of superposing all of the original clock pulse signals on some of the second clock pulse signals wherein the number of the superposed second clock pulse signals is equal to a difference obtained as a result of subtraction of the vertical-pixel count of the display unit from the predetermined vertical-pixel count of the video signal and any adjacent two of the superposed second clock pulse signals pertain to the two kinds of fields separated from each other by at least two horizontal periods; and a gate driving means for receiving the gate clock signals from the gate-clock generating means and for generating gate driving signals used for driving as many gate lines as pulses of the gate clock signals for each horizontal period.
In the display apparatus that functions to display a thinned picture, first of all, a second clock pulse signal for a thinning purpose and original clock pulse signals are generated. The generation of the original clock pulse signals and the second clock pulse signals is repeated for each horizontal period. Then, in order to generate gate clock signals having a pulse count matching the number of vertical pixels of the display unit, the gate-clock generating means generates gate clock signals obtained as a result of superposition of all of the original clock pulse signals on some of the second clock pulse signals wherein the number of the superposed second clock pulse signals is equal to a difference obtained as a result of subtraction of the vertical-pixel count of the display unit from the predetermined vertical-pixel count of the video signal. In the superposition process, a second clock pulse signal is supplied to an inverter serving as a NOT gate. Then, the inverted second clock pulse signal and an original clock pulse signal are supplied to an AND gate to generate a gate clock signal. In this way, gate clock signals are obtained by eliminating as many pulses as inverted second clock pulse signals from original clock pulse signals. Finally, the gate driving means receives the gate clock signals from the gate-clock generating means, generating as many high-level gate driving signals as pulses of the gate clock signals with timings different from each other.
As a result of such operations, some of original clock pulse signals with a vertical-pixel count matching a predetermined horizontal-pixel count of the display unit are eliminated to result in a thinned (shrunk) display with a vertical-pixel count matching the predetermined horizontal-pixel count of the display unit.
In the operations, since a plurality of gate driving signals used for driving the same plurality of gate lines are raised to a high level with timings different from each other, the technique provided by the present invention can be applied to a liquid-crystal display apparatus provided with supplementary capacitors each having a Cs on-gate structure without problems as is the case with the effect exhibited by the function of displaying an enlarged picture.
It should be noted that, in the present invention, the function of displaying an enlarged picture and the function to display a thinned picture respectively enlarge and shrink a picture in the vertical direction and do not enlarge and shrink a display in the horizontal direction.
The display apparatus provided with a function of displaying a thinned picture also has a configuration wherein all of original clock pulse signals are superposed on some of second clock pulse signals with any adjacent two of the superposed second clock pulse signals pertaining to the two kinds of fields separated from each other by at least two horizontal periods for the same reason as the operation of displaying an enlarged picture. If any adjacent two of the superposed second clock pulse signals pertain to the two kinds of fields not separated from each other by at least two horizontal periods, for example, the two kinds of fields generated in two adjacent horizontal periods, video signals of an odd field and an even field appearing on adjacent gate lines will be thinned.
With video signals of for example a slanting line on adjacent gate lines thinned, the thin portions on the adjacent gate lines look striking, appearing as a broken line to the eyes of the user. In the case of the present invention, on the other hand, since any adjacent two of the superposed second clock pulse signals pertain to the two kinds of fields separated from each other by at least two horizontal periods, thinned portions of superposed second clock pulse signals pertaining to an odd field and an even field are distributed on gate lines separated from each other by at least two horizontal periods. As a result, broken portions of the slanting line are not seen by the user, making it possible to obtain a smooth picture.
In general, the driving technique adopted by the display apparatus is a line-sequential driving method which sequentially drives a plurality of gate lines from the top of the screen to the bottom one line after another. An alternative driving technique is an interlace driving method whereby a frame is divided into even and odd fields which are driven alternately by jumping from a gate line to another. The line-sequential driving method includes a technique whereby a frame is divided into even and odd fields and adjacent gate lines in a horizontal period are driven so as to drive all gate lines at a double speed. The present invention is specially suitable for a display apparatus adopting the double-speed line-sequential driving method.
According to the double-speed line-sequential driving method, since two gate lines are driven in a horizontal period, the original clock pulse signal provided by the present invention has a format comprising two pulses per horizontal period. Thus, particularly in the case of an operation to display a thinned picture, by merely eliminating one of the two pulses in appropriate horizontal periods, it is possible to easily generate a gate clock signal having pulses the number of which matches the number of horizontal pixels on the display unit. It is of course that the double-speed line-sequential driving method can also be applied to an operation to display an enlarged picture.
From this point of view, the double-speed line-sequential driving method can be applied to not only a TFT-type liquid-crystal display device but also liquid-crystal display devices of other systems. For example, the double-speed line-sequential driving method can be adapted to an STN-type liquid-crystal display device. Since the response speed of an STN-type liquid-crystal device is low in comparison with the frame frequency of the NTSC or PAL system, however, in the present state of the art, it is considered to be unsuitable for video displays of systems such as the NTSC and PAL systems. Nevertheless, applications to a ferroelectric liquid-crystal display (FLCD) and an antiferroelectric liquid-crystal display (AFLCD) each having a high response speed are possible.
In addition, in order to implement either the function of displaying an enlarged picture or the function of displaying a thinned picture, in a gate-clock generating means, it is desirable to allocate timings to superpose a second clock pulse signal on an original gate clock signal in a horizontal period uniformly to vertical pixels on the display unit.
By having such a configuration, it is possible to obtain a picture with a uniform quality across the entire screen of the display unit.
As described above, the display device provided by the present invention can be applied to a liquid-crystal display apparatus provided with supplementary capacitors each having a Cs on-gate structure without problems. Thus, with the display device of the present invention applied to a TFT-type liquid-crystal display apparatus, each pixel of the display unit employed in the TFT-type liquid-crystal display apparatus can be provided with an electric-charge accumulating capacitor comprising a gate line and a pixel electrode for the pixel.